The present invention relates to a rolling bearing for protection of a magnetic bearing.
A magnetic bearing is used to support in a noncontact state a rotatable shaft of an apparatus such as turbo molecular pump, machine tool, rotating at a high speed, and a rolling beading is used together with the magnetic bearing such that upon the stoppage in function or the magnetic bearing, the rotatable shaft is supported by the rolling bearing.
Conventionally known as such a rolling bearing for protection is for example a structure arranged in the main body of a turbo molecular pump together with magnetic bearings as shown in FIG. 3, where air intake and exhaust are shown by arrows.
The turbo molecular pump has a rotor 3 of the integral type wherein a rotary blade 1 is made integral with a rotatable shaft 2 such that the rotary blade 1 rotates at a high revolution of several ten thousands rpm which produces a peripheral velocity corresponding to the kinetic speed of gas molecules. The turbo molecular pump further has a stationary blade 4, corresponding to the rotary blade 1, which is fixed to the inner wall of a stationary portion or housing 5.
A pair of radial magnetic bearings 7, 8 are provided such that a drive motor 6 is arranged between the radial magnetic bearings 7, 8. The rotatable shaft 2 is rotated by the drive motor 6 and supported in a noncontact state in a radial direction with reference to the stationary portion by the magnetic force of the pair of radial magnetic bearings 7, 8, and in a floating state in an axial direction by the magnetic force of a thrust magnetic bearing 9 at a lower end portion.
However, when it experiences any abnormal accident for example power supply interruption, the magnetic bearings 7, 8, 9 lose their function, so that they may be brought into contact with the rotatable shaft which still rotates at a high speed. It will cause any damages in the magnetic bearings 7, 8, 9. In order to avoid this, a pair of rolling bearings 10, 11 are additionally provided for protection at upper and lower positions.
In the protection bearings 10, 11, clearances between their inner ring and the rotatable shaft 2 are provided, such that the clearances are sized usually in the order of {fraction (1/10)} mm, which are smaller than the clearances of the radial magnetic bearings 7, 8, 9. In the state where the magnetic bearings function in a normal condition, the rotatable shaft 2 never comes into contact with the inner ring of the protection bearings 10, 11.
When the magnetic bearings lose their function, the protection bearings 10, 11 come into contact with the rotatable shaft 2 which rotates at a high speed so as to receive the rotatable shalt 2 for stopping, which is referred to as xe2x80x9ctouch-downxe2x80x9d.
Conventionally used on the thrust load side or for positioning in the axial direction is a protection bearing as shown with reference numeral 11 in FIG. 3 which is a combination bearing comprising a pair of single row, angular ball bearings juxtaposed and used in a dry condition or solid lubrication condition.
When any abnormal state such as power supply interruption occurs, the protection bearing comes into contact with the rotatable shaft 2 rotating at the highest revolution, which causes sudden start of rotation in the protection bearing to rapidly increase the bearing temperature. This rapid temperature increase is outstanding particularly in the case of the turbo molecular pump used in vacuum where there is no cooling action of air convection.
In addition, when the protection bearing used is of a vertical type, whirling load is added thereto due to unbalance, and so the heat generation in the bearing is larger.
In addition, the temperature of the contact surfaces between the bearing balls and the races is higher at local spots, resulting in that plastic flow is easily caused under the contact surface pressure in hertz.
Particularly, the heat generation of the protection bearing causes increase in the load to the rolling members and reduces the radial or internal clearance between the inner race and the rolling members because the contact surface between the inner race of the bearing and the rotatable shaft is smaller than the contact surface between the outer race and the housing so that the temperature of the inner race is higher than that of the outer race. Consequently, the inner race side is subjected to a larger compression stress. On the other hand, the outer race side is subjected to thermal expansion force.
The clearance between the outer race and the housing is sized in a micron order, and so after the temperature on the outer race side becomes high in a degree, there is no clearance between the housing and the outer race, so that the outer race does not expand any more.
On the other hand, since the clearance between the inner race and the rotatable shaft is sized in a order of {fraction (1/10)} mm, it can be easily shrunk.
Supposed that a thrust load Fa is applied downward to the inner race 13 from the rotatable shaft 2 in the protection bearing 11 comprising a pair of single row, angular ball bearings as shown in FIG. 4, and that the thrust load Fa is received by the housing 5 through the outer race 12, such that the load Q, that is compression force, applied to the inner race 13 from the rolling members 14 becomes larger so as to interrupt the expansion force of the inner race 13 due to temperature increase, the inner race 13 is partially yielded. This is advanced because the yield stress decreases due to temperature increase.
As a result, even after the rotation is stopped, the inner race 13, now under the room temperature, is kept shrunk and not restored to its original state. Accordingly, the internal clearances in the radial and axial directions in the bearing are kept larger, and not restored to their original state. This permanent shrinkage is advanced because the plastic flow due to the rolling contact stress on the surface layer portion of the raceways is produced.
An objective of the present invention is to provide a rolling bearing for protection of the magnetic bearing wherein one of the outer and inner races of the rolling bearing is used on the rotatable shaft side such that it is not easily subjected to the yield stress even if a larger load Q from the rolling members between the outer and inner races is applied to the one race upon touch-down, so that the internal clearances in the rolling bearing for protection can be prevented from increasing after it is restored to the room temperature.